The present invention relates to a printer of the fast operation type or gradation print type, in which electric energy is supplied to resistive elements of a print head to effect conversion thereof into thermal energy effective to control print intensity, density and gradation according to the magnitude of the thermal energy.
In the conventional printer utilizing a thermal head, printing is effected line by line such that one line printing is undertaken during several tens ms in response to one or two sequence of a drive pulse and a thermally compensative pulse. The thermal head is provided with a shift register for receiving serial data each line printing. A single counting circuit is also provided to count a number of print dots contained in the serial data to determine a print rate which is a ratio of dots to be printed with respect to the total dot number in one line. Consequently, a drive pulse width is varied each line operation according to the dot numbers to be printed so as to prevent degradation of print quality such as reduction of density and defect of blur, which would occur when the print rate is relatively great.
In the conventional multi-gradation printer, as shown in FIG. 5, several ten times of pulsive drive are carried out each line operation. The print density is determined according to the number and width of drive pulses assigned to individual thermal elements. For example, when 2000 number of dot data are transferred to the thermal head by 4 MHz each time within one line operation, it takes 500 ns to complete the data transfer. Therefore, when effecting 64 times of the data transfer for 64 levels of gradation printing within one line operation, it takes 32 ms to finish each line operation. Such speed is practically too slow.
In view of this, the multi-gradation printer requires provision of a plurality of data transfer circuits for transferring the serial data. However, in such case, a single counting circuit could not measure the print rate as in the above described prior art. The print density would be uneven depending on a number of printed dots without effective compensation for the printing rate.
Further, the fast operation printer such as thermal transfer printer carries out each line printing about 1 ms even without gradation. Therefore, transfer of the serial data cannot be effected in time by a single transfer circuit. Therefore, the fast operation printer likewise requires a plurality of the serial data transfer circuits. Consequently, there is caused the problem that a single counting circuit could not measure correctly the printing rate line by line.